Modeling the time evolution of the dissociation fraction in low-pressure CO2 plasmas

Abstract

The time evolution of the CO2 dissociation fraction in pulsed discharges is studied through kinetic modeling. The simulations are compared against experimental data obtained in pulsed DC glow and radio-frequency discharges, operated with currents of about 40 mA and powers of 40 W, sustained under low gas pressures (< 600 Pa). The model is used to analyse the experimental trends associated with different pulsed configurations, namely different combinations of pulse duration and delay between pulses. The results validate the chemical module used in this work and reveal the importance of electronically excited states, in particular CO(a3Π), to describe the evolution of CO2(X1Σ+) and CO(X1Σg+) densities. In particular, it is shown that CO(a3Π) can promote CO2 formation for increasing concentrations of O2 due to bimolecular reactions such as CO(a3Π) + O2(X3Σg−) → CO2(X1Σ+) + O(3P). At the same time, for low concentration of O2 the CO2 dissociation fraction can be stimulated through CO(a3Π) + CO2(X1Σ +) → 2CO(X1Σg+) + O(3P). It is also found that the pulse parameters influence the concentration of O2(X3Σg−) by favouring or limiting oxygen atomic recombination during the afterglow. Overall, this work addresses reaction mechanisms often overlooked in the CO2 plasma-based reforming literature, while confirming that electronically excited states play a key role in describing the dissociation fraction in CO2 plasmas operated under low pressure conditions.